It has long been known that holograms can be created or written by suitable means such as, for example, by computer-generated signals. In one such system a large multiplicity of points in a volume, designated as x, y, z coordinates, are activated as point sources and coherent light from such point sources is directed to a holographic recording medium for recording in holographic interference with a coherent reference beam. One serious problem in connection with such computer-generated holograms is the immense number of points in a meaningful three dimensional record. For example, it is fully reasonable to wish to record a volume having 500 pixels or several times 500 pixels in each of the x, y, z coordinates and such three dimensional volume would therefore have more than 10.sup.8 pixels. For reconstruction or playback of a hologram recorded from such point sources, the available portion of the dynamic range of the holographic recording material for each image pixel is sharply decreased. As is well known, (a, b, c) sequential exposure of the holographic beam to 10.sup.8 equal exposures reduces the achievable point brightness by a factor of 10.sup. 8 and playback reduces it by another factor of 10.sup.8 so that the point brightness is roughly 10.sup.-16 of that achievable if only a single point is used. This loss comes from the necessity of sharing the total exposure among N separate exposures so that each has only 1/N of the optimum exposure. The resulting minute fraction of the coherent light available for each pixel has stretched the dynamic range of the recording medium beyond its limit and has thus stretched the capability of reconstruction or playback by means of usual coherent sources. The capabilities of recording and reconstruction, to put it simply, are in the order of (1/N).times.(1/N) where each N may be in the range of 10.sup.8 or much worse.
As a result of the forbidding problem of such hologram writing it is usual to pursue different courses and different methods of hologram writing. One such different method is disclosed in a copending U.S. application of Mueller and Caulfield Ser. No. 410,901 wherein a multiplicity of small cells of a hologram are successively recorded as a computer constructed hologram. When such a system is employed for hologram writing with a hologram whose cell may contain for example 200 .mu.m.times.200 .mu.m squares with 2000.times.800 pixels to form a 7 cm.times.7 cm hologram, it may well take a day and a half to write such a hologram at a rate of about three such cells per second.
Quite obviously there remains a need for a hologram construction system to form a hologram for three dimensional hologram recording for appropriate signals such as computer signals and to produce such a holographic recording in a reasonable time such as that generally known in the art as real time.